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<h3>Make diagrams for redox reactions:</h3>

<p>
<ul>
  <li><b><a href="#Fe-corr">Iron corrosion</a></b></li>
  <li><b><a href="#Cu-CN">Reduction of <nobr>Cu(II)</nobr> to 
      <nobr>Cu(I)</nobr> by <nobr>CN<sup>&#8722;</sup></nobr></a></b></li>
</ul>
</p>
<a name="Fe-corr"></a>
<hr><!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->

<h3>Iron corrosion</h3>

<p>Make a diagram that shows the corrosion products formed as a function
of the added amount of Fe metal to water.</p>



<p><b>Step by step instructions:</b>
<b>1)</b> Start <a href="DB_0_Main.htm">DATABASE</a> and click on &#147;Fe&#148;
(iron) in the periodic table. Click on <nobr>&#147;Fe 2+&#148;</nobr>
in the list of available components. Click also on the
<nobr>&#147;e-&#148;</nobr> (electron) button.
The list of <i>components selected</i> will now show <nobr>&#147;H+&#148;,</nobr>
<nobr>&#147;e-&#148;</nobr> and <nobr>&#147;Fe 2+&#148;.</nobr>
Click on the pull-down menu <nobr>&#147;File&nbsp;/</nobr> Save and
<nobr>Exit&#148;.</nobr>
Click on the SPANA button to save the data file and to make a diagram.
Enter a file name, for example <nobr>&#147;Iron&#148;.</nobr></p>

<p><b>2)</b> The <a href="S_0_Main.htm">SPANA</a> main window will
appear. Remember that the chemical system has the components:
<nobr>H<sup>+</sup>,</nobr> <nobr>e<sup>&#8722;</sup></nobr> and
<nobr>Fe<sup>2+</sup>.</nobr> A diagram showing the corrosion products
as a function of added Fe metal is needed and <nobr>Fe(c)</nobr> must be a
component instead of <nobr>Fe<sup>2+</sup>.</nobr> Click on the pull-down menu
<nobr>&#147;Run&nbsp;/</nobr> Modify chemical <nobr>system&#148;.</nobr>
The &#147;modify&#148; window appears. Click on <nobr>&#147;Fe 2+&#148;</nobr>
in the soluble components list and then &#147;Exchange a component with a reaction&#148;
<img src="images/exchange.gif" alt="Exchange button" title="Exchange button" align="texttop" height="37" width="37" border="0"></p>

<p>A new window will appear listing the available reaction products.
Click on <nobr>&#147;Fe(c)&#148;</nobr> in the list of solid products
and click on the <nobr>&#147;Ok&#148;</nobr> button to confirm. Back in
the &#147;modify&#148; window the soluble components are <nobr>H<sup>+</sup></nobr> and
<nobr>e<sup>&#8722;</sup>,</nobr> and <nobr>Fe(c)</nobr> is a solid component.
Click on the <nobr>&#147;Save changes&#148;</nobr> button.
Enter a file name, for example <nobr>&#147;Fe-corr&#148;.</nobr></p>

<p>Back in the main Spana window click on the pull-down menu
<nobr>&#147;Run&nbsp;/</nobr> Make a <nobr>Diagram&#148;.</nobr>
A new window appears to select what type of diagram is wanted.
Change the component in the <i>X</i>-axis to <nobr>&#147;Fe(c)&#148;.</nobr>
To make a diagram that shows the corrosion process in pure water, you must set the
<i>added</i> amounts of <nobr>H<sup>+</sup></nobr> and
<nobr>e<sup>&#8722;</sup></nobr> to zero.
On the <nobr>&#147;<U>C</u>oncentrations&#148;</nobr> frame click on the
<nobr>&#147;pH = 7&#148;</nobr> line, change &#147;pH&#148; for
&#147;Total conc.&#148;, change the value to 0 (zero)
and press Enter. Then, on the same
concentrations frame, click on the <nobr>&#147;pe = 8.5&#148;</nobr>
line, enter a total concentration of zero for <nobr>e<sup>&#8722;</sup></nobr> and press
Enter.</p>

<p>Click on the Diagram button in the upper-left corner of the window.
<center>
<img src="images/Make_diagr.gif" alt="Make diagram" title="Make diagram" height="67" width="64" border="0">
<br clear="all">
</center>
<a name="magnetite"></a>
The calculations will be done by a console program.
When the calculations are finished the diagram is displayed in a window:
<center>
<img src="images/Diagr_LogFecorr.gif" alt="Diagr Fe-corrosion" title="Diagr Fe-corrosion" height="241" width="304" border="1">
</center>
The diagram shows the amounts of corrosion products (in moles per litre of water)
as a function of iron metal reacted (in millimoles of <nobr>Fe(cr)</nobr> per
litre of water). In pure water magnetite is the solid corrosion product:</p>
<p class="indent">
    <nobr>3 Fe(cr)</nobr> <nobr>+ 4 H<sub>2</sub>O</nobr>
		&nbsp;<img src="images/ArrowRt.gif" alt="==>" title="==>" height="9" width="16">&nbsp;
    <nobr>Fe<sub>3</sub>O<sub>4</sub>(cr)</nobr> <nobr>+ 4 H<sub>2</sub></nobr>
    </p>

<a name="Cu-CN"></a>
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<h3>Reduction of <nobr>Cu(II)</nobr> to <nobr>Cu(I)</nobr> by cyanide</h3>

<p>
In a laboratory exercise the students have to react <nobr>Cu<sup>2+</sup></nobr>
and <nobr>Cd<sup>2+</sup></nobr> with <nobr>CN<sup>&#8722;</sup>.</nobr>
The experiment with Cd produces a clear solution while Cu precipitates.</p>
<p><a name="Fig1-2"></a>
Illustrate first the Cd experiment by making a diagram with
<nobr>[Cd]<sub>T</sub></nobr> = 10 mM and varying the
<nobr>CN<sup>&#8722;</sup></nobr> concentration. Assume that the initial
Cd solution had a pH of 4, <nobr><i>i.e.</i></nobr> set the total concentration of <nobr>H<sup>+</sup></nobr>
to <nobr>10<sup>&#8722;4</sup></nobr>
<nobr>(<a href="Tut_Redox_Fig1.htm">Figure</a>).</nobr>
Why is there no precipitate? What would happen
if instead of <nobr>CN<sup>&#8722;</sup> you had added
<nobr>OH<sup>&#8722;</sup></nobr>?</nobr>
<nobr>(<a href="Tut_Redox_Fig2.htm">Figure</a>).</nobr></p>

<p>In the case of <nobr>copper(II)</nobr> a redox process takes place:</p>
<p class="indent">
    <nobr>Cu<sup>2+</sup></nobr> + <nobr>CN<sup>&#8722;</sup></nobr>
            + <nobr>e<sup>&#8722;</sup></nobr>
		<img src="images/rlh.gif" alt="=" border="0" width="14" height="10" border="0">
    <nobr>CuCN(s)</nobr><br>
    <nobr>CN<sup>&#8722;</sup></nobr>
		&nbsp;<img src="images/ArrowRt.gif" alt="-->" title="-->" height="9" width="16" border="0">&nbsp;
    &frac12; <nobr>(CN)<sub>2</sub>(g)</nobr> + <nobr>e<sup>&#8722;</sup></nobr>
    </p>
<p>Make a diagram to study what happens. Select the components: electron,
<nobr>CN<sup>&#8722;</sup></nobr> and <nobr>Cu<sup>2+</sup>.</nobr>
If you select both <nobr>Cu<sup>2+</sup></nobr> and <nobr>Cu<sup>+</sup></nobr>
as components, you can model the solubility, but not the redox process.
Use the same concentrations as in the Cd case. To simulate the redox process,
set the total concentration of added electrons to zero,
<nobr>[e<sup>&#8722;</sup>]<sub>T</sub></nobr> = 0.</p>

<p>
<b><a href="Tut_Redox_CuCN.htm">Step by step instructions</a></b>
</p>

<p>The diagram:
<center>
<img src="images/Diagr_FracCuCN.gif" alt="Diagram Fract Cu-CN" title="Diagram Fract Cu-CN" height="241" width="304" border="1">
</center>
There is an &#147;ideal&#148; <nobr>CN<sup>&#8722;</sup></nobr> concentration
where almost all Cu precipitates. If an excess of ligand is added soluble
<nobr>Cu(I)</nobr> complexes are formed and the solid redisolves. 
</p>

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